Credit Facilities - Credit Facilities and Financing Bridge Commitment (Details)	Dec. 31, 2023 USD ($) lender	Sep. 30, 2023 USD ($)
Line of Credit Facility [Line Items]
Uncommitted Credit Facilities	$ 67,100,000	$ 55,500,000
Note Payable to Bank	7,400,000	7,500,000
Senior Secured Notes	342,900,000	342,100,000
Total outstanding borrowings	761,400,000	683,100,000
Secured Debt | Senior StoneX Group Inc. Committed Credit Facility - Revolving Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing	$ 42,500,000
Number of lender | lender	1
Senior Notes
Line of Credit Facility [Line Items]
Debt instrument, unamortized discount (premium) and debt issuance costs, net	$ 5,100,000	5,800,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facilities	1,200,000,000
Amount outstanding	344,000,000.0	278,000,000.0
Revolving Credit Facility | Senior StoneX Group Inc. Committed Credit Facility - Revolving Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing	500,000,000.0
Long-term line of credit, noncurrent	253,000,000.0	150,000,000.0
Revolving Credit Facility | StoneX Financial Inc.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	190,000,000.0
Line of credit facility	0	0
Revolving Credit Facility | StoneX Commodity Solutions LLC
Line of Credit Facility [Line Items]
Line of credit facility borrowing	400,000,000.0
Line of credit facility	51,000,000.0	103,000,000.0
Revolving Credit Facility | StoneX Financial Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	100,000,000.0
Line of credit facility	40,000,000.0	25,000,000.0
Revolving Credit Facility | StoneX Financial Pte. Ltd.
Line of Credit Facility [Line Items]
Line of credit facility borrowing	10,000,000.0
Line of credit facility	$ 0	$ 0